Correctly passing an array from a function - c

Question

I have some code to generate an array of size [user_input] in a function called array_generator, using size of array from scanf in main(), and then filling it with the numbers 0 to user_input (0, 1, 2, 3, if user input is 4). The array fills correctly as printf prints
`The array contains the value 1`
`The array contains the value 2`
`The array contains the value 3`, etc.
However when I pass the array to main and printf the array values I get equal statements filled with garbage numbers. I'm also 90% sure I have been passing the arrays and pointers incorrectly (new to them).
The code is below:
#include <stdio.h>
int *array_generator(int number_songs);
int main(void)
{
int input;
int *p;
int i;
int x;
printf("Enter number of songs wanted in random playlist: ");
scanf("%d", &input);
p = array_generator(input);
x = *p;
for (i = 0; i < input; i++)
{
printf("The array contains the values %d\n", x);
}
return 0;
}
int *array_generator(int n)
{
int a[n];
int *p;
int i;
for (i = 0; i < n; i++)
{
a[i] = i;
printf("The array contains the values %d\n", i);
}
return p = &a[n];
}

Answer 1

One simple solution is to define an array which is bigger than the largest list of songs reasonably possible. For example, since you print every entry, more than a few hundred are not reasonable. On a modern computer space is abundant. You would have a define for the max size on top of the prog, or later in some header:
#define MAX_SONGLIST_LEN 1000
The array can be global, or it can be static inside the function. Let's make it static because you want the function to return the address.
The change is minimal. Just say
static int a[MAX_SONGLIST_LEN];
You may want to change the loop and check for the max length as well:
for (i = 0; i < input && i < MAX_SONGLIST_LEN; i++)
inside array_generator() and main(). You also may want to inform your users about the maximum, and catch numbers which are too large. (You don't do any error handling of user input anyway -- what happens if the user enters a letter instead of a number? Look into the return value of scanf().)
The static array's life time is the lifetime of the program. It will be initialized to all zeroes by the way. If you want to randomly initialize it look at the rand() function.

Answer 2

You are correct in that you are using pointers wrong. The code below preforms the function that you want...
#include <stdio.h>
void array_generator(int n, int arr[]) {
for (int i = 0; i < n; i++) {
arr[i] = i;//array values
//printf("The array contains the values %d\n", i);
}
}
int main() {
int input;
printf("Enter number of songs wanted in random playlist: ");
scanf("%d", &input);
int array[input];//declare array with length of input
array_generator(input, array);
for(int i=0; i<sizeof(array)/sizeof(array[0]); i++) {//input could also be used at the limit to the for loop
printf("%d", array[i]);
}
printf("\n");
return 0;
}
What you are doing is you are over complicating your code. The first thing you do is you create a function and try to make it return an array. This is not necessary. All you need to do is pass a pointer to the array and all edits to the array will be made on the same scope as the array was declared on.
You also mentioned that you want to find the size of an array. This can be done with array *a of any type and sizeof(a)/sizeof(a[0]). This works by returning the number of bytes used by the array divided by the number of bytes used by the first element in the array.
One more thing that you have that you don't need is x = *p;. when you do this, you are essentially doing this x=*p=array_gen(input);
More information on pointers in C can be found here.